viruses Review A Current Update on Human Papillomavirus-Associated Head and Neck Cancers Ebenezer Tumban Department of Biological Sciences, Michigan Technological University, 1400 Townsend Dr, Houghton, MI 49931, USA; [email protected]; Tel.: +1-906-487-2256; Fax: +1-906-487-3167 Received: 16 September 2019; Accepted: 4 October 2019; Published: 9 October 2019 Abstract: Human papillomavirus (HPV) infection is the cause of a growing percentage of head and neck cancers (HNC); primarily, a subset of oral squamous cell carcinoma, oropharyngeal squamous cell carcinoma, and laryngeal squamous cell carcinoma. The majority of HPV-associated head and neck cancers (HPV + HNC) are caused by HPV16; additionally, co-factors such as smoking and immunosuppression contribute to the progression of HPV + HNC by interfering with tumor suppressor miRNA and impairing mediators of the immune system. This review summarizes current studies on HPV + HNC, ranging from potential modes of oral transmission of HPV (sexual, self-inoculation, vertical and horizontal transmissions), discrepancy in the distribution of HPV + HNC between anatomical sites in the head and neck region, and to studies showing that HPV vaccines have the potential to protect against oral HPV infection (especially against the HPV types included in the vaccines). The review concludes with a discussion of major challenges in the field and prospects for the future: challenges in diagnosing HPV + HNC at early stages of the disease, measures to reduce discrepancy in the prevalence of HPV + HNC cases between anatomical sites, and suggestions to assess whether fomites/breast milk can transmit HPV to the oral cavity. Keywords: HPV; oral transmission; head and neck cancers; HPV vaccines; HIV and AIDS; head and neck cancer treatment 1. Introduction Head and neck cancers (HNC) account for ~4.8% of cancers and they are associated with a similar percentage of cancer mortality worldwide [1]. Ninety percent of HNC arises from squamous epithelial cells lining the oral cavity, pharynx, larynx, or, more rarely, the nasal cavity. These include: (i) oral squamous cell carcinomas (OSCC), which are cancers that arise from lips, tongue, floor of the mouth, oral cavity, etc.; (ii) oropharyngeal squamous cell carcinomas (OPSCC), which are cancers that arise from the base of the tongue, the soft palate, tonsils, back of the throat; (iii) laryngeal squamous cell carcinomas (LSCC), which arise from the supraglottis, glottis, subglottis; (iv) nasal squamous cell carcinomas (NSCC; to a lesser extent), which arise from squamous epithelial cells lining the nasal cavity and paranasal sinuses [2–4]. The remaining 10% of HNC arises from lymphocytes, cells of connective tissue (muscle, blood vessel), and cells of the salivary glands [3]. Many factors/co-factors have been linked to HNC: alcohol consumption [5,6], smoking, and/or chewing of tobacco [7,8] increase the risk for HNC. Alcohol consumption is associated with ~5% of HNC cases, tobacco use is associated with ~34%, while consumption of alcohol in combination with tobacco use is associated with 36% of ~HNC cases [5,7,9]; thus, ~75% of HNC (i.e., squamous cell carcinomas) are caused by alcohol and tobacco use. The remaining percentage (~25%; worldwide average) of HNC cases is caused by human papillomaviruses (HPVs) [10,11]. HPVs also cause almost all cases of cervical cancer, a percentage of other anogenital cancers (vaginal, vulva, anal, penile, etc.), and almost all cases of genital warts [12]. This review focuses only on HPV-associated HNC (HPV + HNC): it summarizes the transmission Viruses 2019, 11, 922; doi:10.3390/v11100922 www.mdpi.com/journal/viruses Viruses 2019, 11, 922 2 of 19 of HPV to the head and neck region, the contribution of HPV to HNC (specifically OSCC, OPSCC, and LSCC). It also highlights measures currently being used (with potentials) to protect against oral transmission of HPVs as well as challenges facing the diagnosis and treatment of HPV + HNC. 2. HPV, Genome, and Oncogenes More than 220 HPV types have been identified as of 2019 [13]. HPVs are non-enveloped double-stranded DNA viruses with a circular genome of ~8000 base-pairs; HPVs infect and replicate in epithelial cells in the skin and mucosal regions [14]. Its genome codes for six nonstructural genes (early genes: E1, E2, E4, E5, E6, and E7) and two structural genes (late genes: L1 and L2) (Figure1). L1 and L2 are required for viral assembly while E1, E2, and E4 are required for viral replication and regulation of replication; E5, E6, and E7 on the other hand are oncogenes and are implicated in HPV-associated cellular transformation [14,15]. Following HPV infection, the genome is maintained as an episome in the nucleus; integration of E6 and E7 into the host chromosome and persistent expression of these oncogenes interferes with the functioning of cell cycle regulator (tumor suppressor) proteins. E6 binds to p53 via E6AP (also known as ubiquitin-protein ligase E3A; UBE3A) and targets p53 for degradation via a proteasome-mediated pathway [16]. E6 and to some extent E7 have also been reported to enhance telomerase activity especially its catalytic unit, human telomerase reverse transcriptase (hTERT; elongates telomeres); reviewed in ref. [17]. Under normal circumstances, hTERT is constitutively repressed in somatic cells. However, E6 removes transcriptional suppressors from the promoter of hTERT and hypomethylates and acetylates the promoter; reviewed in ref. [17]. These changes cause the expression of hTERT, which then elongates telomeres (via replication) thus preventing end-to-end fusion of chromosomes (crisis) and apoptosis [18]; this provides an opportunity for the cell to continue to divide and become immortal. E7, on the other hand, binds to pRb protein, dissociating it from E2F (a transcription factor), and thus E2F drives the cell to enter, uncontrollably, the S phase of the cell cycle [19]. The inability of these cell cycle regulators to control cell division leads to uncontrolled cell proliferation and ultimately cancer [20]. In addition to p53 and pRb inactivation, other cancer-related pathway proteins (independent of p53 and pRb), especially those involved in regulating the cell cycle, are also targeted by E6 and E7 oncoproteins. For example, E7 binds and inactivates cell-cycle regulatory proteins such as inhibitors of the cell cycle (cyclin-depend kinase inhibitors: p21CIP1 and p27Kip1) while at the same time, it binds to cyclin-dependent kinase 2 and enhances its activity [20–22]; both events promote cell division. Furthermore, E6 and E7 oncoproteins also target, indirectly, non-coding RNAs (known as microRNAs; miRNA) associated with tumor progression/suppression. For example, E6 and E7 increase the levels of miRNA-21 (oncogenic miRNA), which negatively targets the expression of PTEN (a tumor suppressor gene) [23,24]. At the same time, E5, E6, and E7 expression in cervical cancer cell lines (CaSki and siHa) downregulate the expression of some microRNAs (tumor suppressive miRNAs: miRNA-22, miRNA-148a-3p, miRNA-190a-5p, miRNA-450, miRNA-455, and miRNA-203), which control/inhibit cell proliferation and as such, the oncoproteins promote cell proliferation [24–26]. Thus, HPV oncogenes enhance/decrease miRNA levels and the effect of the miRNAs on HPV-associated cancer depends on target genes (tumor suppressor genes versus proto-oncogenes). It is also worth mentioning that E6 and E7 have other functions, in addition to serving as oncogenes, which help reprogram the cell to enhance the replication of HPV (e.g., antiviral responses). For example, E6 and E7 bind to interferon regulatory factor-3 protein and interferon regulatory factor-1 protein, respectively, and inactivate these proteins; this thus, inhibits interferon signaling pathways and allows the virus to evade the immune system [21]. In addition to these, E6 binds to death receptor (FADD) on the cells, thus blocking fas-mediated apoptosis [20–22]. This allows the cells to serve as a “factory” for the virus to continue to replicate. Viruses 2019, 11, 922 3 of 19 Viruses 2019, 11, x FOR PEER REVIEW 3 of 18 FigureFigure 1. 1A. A schematic schematic ofof the genome of of HPV16 HPV16 and and capsid capsid proteins. proteins. Right Right image: image: genes genes that code that for code forearly early proteins proteins (E1, (E1, E2, E2, E4, E4, E5, E5, E6, E6, and and E7), E7), are are shown shown in inred red and and green green colors; colors; E5, E5, E6, E6, and and E7 E7are are oncogenes.oncogenes. Genes Genes that that code code forfor latelate proteins (L1 and and L2; L2; capsid capsid proteins) proteins) are are shown shown in inlight light blue blue color. color. URRURR (upstream (upstream regulatory regulatory region) region) contains contains origin origin of replication, of replication, enhancer enhance elements,r elements, and early and promoter early promoter (p97); URR controls viral replication. p670 is the late promoter. pAE and pAL are early (p97); URR controls viral replication. p670 is the late promoter. pAE and pAL are early polyadenylation polyadenylation and late polyadenylation sites, respectively. Left image: The L1 protein forms and late polyadenylation sites, respectively. Left image: The L1 protein forms pentamers (one is circled) pentamers (one is circled) and each pentamer has an L2 protein at its center (not shown). Seventy-two and each pentamer has an L2 protein at its center (not shown). Seventy-two copies of the pentamers copies of the pentamers assemble to form an icosahedral capsid. assemble to form an icosahedral capsid. It is also worth mentioning that E6 and E7 have other functions, in addition to serving as E5 oncoprotein is believed to enhance the cellular transformation by activating epidermal growth oncogenes, which help reprogram the cell to enhance the replication of HPV (e.g., antiviral factor receptors on epithelial cells [20,22]. responses). For example, E6 and E7 bind to interferon regulatory factor-3 protein and interferon HPVs are divided into two groups based on their association with neoplasia: low-risk types regulatory factor-1 protein, respectively, and inactivate these proteins; this thus, inhibits interferon and high-risk types.